This invention relates to the generation of electrical power. More particularly, it relates to integrated automatic control of electrical power generating stations.
Due to the extreme size, complexity, and expense of power generating equipment, effective operational control is an absolute necessity. The continuing increase in demand for electric power, unanticipated delays in provision for new generating capacity additions, the trend toward larger generating stations, and increased interconnection dependency among power companies are among the many factors which have magnified the importance of individual unit response capability to power system operating objectives. Not only are generating systems expected to provide a given demand with reasonable consistency, but exceptional demands may also require rather rapid adaptation of output capacity to achieve a given megawatt demand. Realization of these goals, however, has been severely limited by the scope of available technology.
A typical power generating unit involves substantial mechanical complexity. A boiler unit, fed by water for steam and fuel and oxygen for heating, includes one or two separate furnaces and a complete water circulation system. The steam produced by the boiler is passed through one or more superheaters and thence to the throttle valves of a turbine governing stage. After being passed through a high pressure turbine, the steam can be reheated and passed again through a second turbine. All stages of this apparatus involve distinct physical characteristics. Moreover, at many points through the system, discrete controls afford an opportunity for changing one or more of those characteristics. Problems occur, however, because virtually all characteristics and controls throughout the cycle are interdependent upon one another, such that change of virtually any control parameter has physical effects at disparate points through the system. For example, in order to increase output production, a first step features opening a governing valve to the turbines. This, however, affects the pressure of steam in the superheaters (i.e., throttle pressure) and the level of water in the drum of the boiler (i.e., drum level). In order to keep throttle pressure up to a desired level, fuel valve positions in the boiler may be changed. This however, produces added heat to the drum, and the position of the feedwater valve to the boiler is subsequently manipulated. With the altered flow of water into the boiler, however, other factors are affected, and the cyle continues.
The very complex interaction of all these internal factors in generating units makes it difficult to realize rapid changes in megawatt output.
The preponderance of prior art systems tend to isolate several more or less distinct control aspects of a system, and to treat them on a strictly individual basis, For example, turbine throttle pressure has been linked rather directly to the position of the fuel valves to the boiler. Likewise, output power has been associated with the position of the governing valve of the turbine. This segmented approach to control achieves a certain amount of stability in that, under a limited output demand, operational stability can be achieved. Nevertheless, the treatment of a complex, integrated system as a series of discrete subsystems is quite deficient in at least two respects. First, the subsystems approach requires some degree of approximation which ignores certain interactions. Secondly, lack of provision for interaction among the discrete subsystems renders it extremely difficult to meet changing generation demands.
In order to solve these difficulties, recent control systems have attempted to provide limited coordination between the discrete subsystems. First, use of variable set points for the discrete subsystem input quantitites has been suggested. Also, a limited degree of coordination between power generation and throttle pressure subsystems has been shown. Finally, proposal has been made to add feed forward capabilities of input quantities to achieve further compensation facility. The former two additions have been applied to discrete subsystems, but the feed forward provision has generally been found to be impractical for the reason that calibration and set-up of the controls is both time consuming and expensive. Additional feed forward apparatus necessitates complete revision of well-accepted calibration techniques.
Sporadic attempts to integrate the control subsystems of power generating systems have thus far proven unsatisfactory. In fact, at least one expert has concluded that integrated control schemes would not in any event significantly improve the performance of the systems.
It is a primary object of the present invention to afford integrated optimal control which allows for complete consideration and interaction of all appropriate system parameters.